Alternating current electric machines, such as motors and generators, may be operated with a frequency converter. Frequency converter may be connected between a power line and the electric machine. The power line may be further connected to a power grid. Frequency converter may include an inverter which provides variable voltage and variable frequency supply voltage to the windings of the alternating current electric machine. Frequency converter may also include a line bridge which supplies electric power to/from the power line.
When operating, a magnetic flux wave travels in a magnetic circuit inside the electric machine. Magnetic flux wave normally includes (sinusoidal) fundamental wave and also some harmonics. These harmonics may be caused by non-sinusoidal rotor magnetization, such as non-sinusoidal magnetic field of permanent magnets of a permanent magnet synchronous machine. Harmonics may also be caused by discrete placing of the windings, for example in electric machines having fractional slot concentrated winding. Permanent magnet synchronous machines having fractional slot concentrated windings are used nowadays in elevator drives and wind turbine generators, for example.
The harmonics of the alternating current electric machine may have effect that electrical power is supplied irregularly to/from the electric machine. Irregular supply of power further causes distortion in line current. Distortion in line current is unwanted, because it causes disturbances in the power grid, therefore having the effect that the capacity of the power grid is not used in the most efficient way.